1. Field of the Invention
The present invention relates to an optical transceiver that is applied to an optical network such as a Passive Optical Network (PON) and a Subcarrier Multiplexing over Wavelength Division Multiplexing (WDM) PON, and more particularly to an optical transceiver for transmitting light source control information and an optical network using the same, which are implemented so as to allow an Optical Network Terminal (ONT) at the subscriber side (hereinafter referred to as a “subscriber ONT”) and/or an Optical Line Terminal (OLT) at the communication company side (hereinafter referred to as a “telephone office OLT”) to transmit light source control information by incorporating it into an SCM frame, which is a physical layer transport frame, in an SCM optical network, so that information of Optical Beat Interference (OBI) on a communication link can be transmitted in real time in the optical network such as a PON, thereby making it possible to manage and control a light source causing OBI.
2. Description of the Related Art
The most important factor in development of optical network technology is development of a transmission technique that is cost-effective and supports mass production, taking into consideration characteristics of a subscriber access network. To accomplish this, it is necessary to reduce the price of optical parts and also to provide a technique for accommodating a large number of subscribers. One method of implementing such an economical optical communication system is to allow a large number of subscribers to share wavelengths and increase the number of subscribers, given a set band of wavelengths.
One method of increasing the number of subscribers is a Subcarrier Multiplexing technique, in which light sources of subscribers with different subcarriers share a wavelength. In this technique, a subscriber incorporates its information into a subcarrier assigned to the subscriber to transmit the information, and the receiving side uses a band pass filter corresponding to the subscriber to pass a signal received from the subscriber to extract the information of the subscriber.
One conventional SCM optical communication system is described below with reference to FIG. 1.
FIG. 1 is a schematic block diagram of the conventional SCM optical communication system.
As shown in FIG. 1, the conventional SCM optical communication system comprises a plurality of subscriber ONTs 10-1 to 10-N including a plurality of optical transceivers 11-1 to 11-N for transmitting a plurality of optical signals, respectively, through a single wavelength, an optical coupler 20 for coupling the optical signals, transmitted from the optical transceivers 11-1 to 11-N of the subscriber ONTs 10-1 to 10-N, to a single optical fiber, and a telephone office OLT 30 having an optical transceiver 31 for receiving an optical signal output from the optical coupler.
In the conventional optical communication system shown in FIG. 1, the same wavelength is used for transmission from the subscriber ONTs 10-1 to 10-N to the optical coupler 20, but information of the subscriber ONTs 10-1 to 10-N is carried on different subcarriers λ1.1 to λ1.n. In this manner, the SCM technique allows a plurality of subscribers to share a single wavelength, lowering network implementation costs. Thus, the SCM technique makes it possible to implement a low-cost optical network.
FIG. 2 is a block diagram of one of the conventional optical transceivers shown in FIG. 1.
The optical transceiver shown in FIG. 2 comprises an electrical-to-optical/optical-to-electrical (EO/OE) converter for electro-optically converting an electrical signal for transmission into an optical signal and providing the optical signal to an optical fiber, and photoelectrically converting an optical signal received through the optical fiber into an electric signal.
As is known in the art, in an SCM-based optical network, optical beat interference (OBI) occurs if an optical transceiver located at a telephone office OLT simultaneously receives at least two optical signals from subscriber ONTs. The central frequency of OBI noise corresponds to the difference between the central frequencies of two received optical signals, and the spectrum of the OBI noise has a form similar to that of the convolution of the spectrums of the two optical signals.
The frequency of the OBI noise may be present in a band of subcarrier signals in the photoelectric conversion procedure so that the OBI noise is a major factor decreasing the signal to noise ratio of electrical signals produced by the photoelectric conversion. Thus, there is a need to perform appropriate measurement, transmission and control of the OBI noise.
In other words, if a frequency corresponding to the difference between the central frequencies of two received optical signals is present in the band of subcarrier signals, OBI occurs in the band of subcarrier signals, and the OBI serves as noise, reducing the signal to noise ratio. Thus, it is necessary for the SCM based optical network to reduce the OBI noise. As another example, if an optical receiver receives two optical signals that have the same optical spectrum and have nearly the same central frequency, the optical receiver has an electrical spectrum as shown in FIG. 3.
FIG. 3 shows a received signal spectrum for illustrating OBI noise occurring in the conventional optical transceiver. In FIG. 3, if the width Δλ of the total wavelength range is near zero, i.e., if the two optical signals have nearly the same central frequency, the central frequency of the OBI noise is near zero as can be seen from the received signal spectrum of FIG. 3. This is because OBI noise occurs at a frequency corresponding to the difference between the central frequencies of the optical signals. If the optical signals have a narrow spectrum, substantial OBI occurs in the narrow frequency band about a frequency of zero so that the OBI serves as a major source of noise in the subcarrier band.
One conventional method of reducing OBI noise is described in, “Optical Beat Noise Suppression and Power Equalization in Sub Carrier Multiple Access Passive Optical Networks by Downstream Feedback” (Journal of Lightwave Technology, Vol. 18, No. 10 October 2000, p 1337-p 1347). In this method, a band other than frequencies used to modulate optical signals is selected, and only the selected band is passed through a filter to measure noise. A Central Processing Unit (CPU) of an OLT transmits light source control information through an electrical monitoring channel to control the wavelength of a light source causing OBI.
Another conventional method of reducing OBI noise uses a light source, into which a temperature control module and a laser diode are packaged. In this method, the temperature of laser diodes of different light sources is controlled to separate the central frequencies of the different light sources far apart from each other so that OBI occurs at a frequency above the subcarrier band.
However, the conventional OBI reduction methods reduce OBI by directly controlling the light source or using a device for controlling the light source. Since OBI occurs at the optical transceiver of the telephone office OLT and the OBI is controlled in the subscriber ONT, the conventional OBI reduction methods have the following problems. In order for the telephone office OLT to monitor and control OBI of a subscriber ONT in the SCM optical network, the subscriber ONT must transmit its light source information to the telephone office OLT, and the telephone office OLT must transmit light source control information required to control the light source of the subscriber ONT to the subscriber ONT through a separate communication path.
After detecting the occurrence of OBI, the OLT must quickly transfer light source control information required to control the light source of the ONT to the ONT through a communication path established between the OLT and the ONT. In addition, both the OLT and the ONT need to have a function to control the light source of the ONT without changing the existing commercial communication architecture or the existing user interface.